Debates don’t move polls. Debate winners do.

David Rothschild, Yahoo! News

Economist

The SignalOctober 4, 2012

Mitt Romney won the first debate; virtual every snap poll and snap pundit agrees on this point. As the 90-minute debate wore on, the Republican challenger's odds of unseating President Barack Obama rose about 5 percentage points to 31 percent in the Signal's election model, driven by gamblers who dumped the president's stock during and immediately after the faceoff.

We are unlikely to see as large a movement in the polls, at least right away. By themselves, debates seldom move polls precipitously. Political scientist Thomas Holbrook has found that the average change over the last 16 presidential debates is less than 1 percentage point. The maximum change was 2.26 percentage points before and after the first debate in 2004, when John Kerry came out swinging against George W. Bush.

Sources: Betfair, Intrade and IEM

But debates have a reach beyond the immediate bump or slide in the polls as they seep into the narrative and offer up ammunition for campaign commercials. With nearly two full weeks until the next presidential debate, the results of this one have a long time to hang around. Romney's solid performance can lead to new donations that, in turn, lead to better poll numbers in the following weeks.

In this way, debates are the opposite of conventions, in which we advise you to ignore the bump in the polls since it inevitably fades. After debates, we advise you to ignore the nonbump in the polls, because it may grow.

Our prediction moved in Romney's favor because, with the wind in his sails, he is slightly more likely to be able to close the 4.5 percentage point gap in the polls over the next few weeks. That remains, so to speak, an uphill sail.

David Rothschild has a Ph.D. in applied economics from the Wharton School of Business at the University of Pennsylvania. Follow him on Twitter @DavMicRot.